The objective of the proposed research is a) to ascertain the relationship between various yeast species by use of molecular biological technques; b) to investigate various strains and species whose taxonomic positions are obscure because of the lack of finite characteristics and the inadequacies of the current classification system; c) to establish sound criteria for species differentiation to be used to evaluate the relatedness of former and new strains, and d) to provide a basis for a more meaningful taxonomy of the yeasts. Yeast DNA will be evaluated for a) base composition, b) genome size, c) amount of repetitive DNA and d) degree of nucleic acid reassociations (DNA-DNA and DNA-RNA hybridizations). These parameters will be used to establish sound criteria for species differentiation, to evaluate relatedness of former and new strains of yeasts, and to provide a basis for a more meaningful taxonomy of yeasts.